1. Field of the Invention
The present invention pertains to humidifiers of the type which are designed to be incorporated within a heating, ventilation and air conditioning ("HVAC") system. More particularly, the invention relates to steam-type humidifiers which use a heat exchanger to transfer thermal energy from a first steam source to heat water into steam.
2. Description of the Prior Art
Steam to steam type humidifiers such as the Model STS.TM. brand humidifier manufactured by Dri-Steem Humidifier Company of Eden Prairie, Minn. utilize a heat exchanger to transfer thermal energy from a first steam source to heat water into steam. Such systems allow waste steam which would be unfit for humidification uses to be used to produce a cleaner steam that is free from salts and other impurities which are likely to be found within conventional boiler generated steam.
The heat exchangers in such systems typically have several parallel branches, in order to maximize heat transfer between the steam and the water which is intended to be heated. The parallel heat exchanger branches are usually connected to common supply and return manifolds, which in turn are connected to a supply line having an automatic steam valve, and a return line having a steam trap, respectively.
Efficiency in such a system is maximized when the steam flows in an evenly distributed manner into the parallel heat exchanger branches. Unfortunately, this has proved somewhat difficult to achieve in practice, primarily because air tends to get trapped in one or more of the parallel branches when steam is introduced to the heat exchanger after a shut-down period during which the heat exchanger fills with air.
The present inventor believes that the problem is caused by unequal flow of the steam in the various branches, as it initially fills the heat exchanger after shut down.
When the steam valve in such a system first opens, the heat exchanger tubes are filled with air and the thermostatically controlled air valve inside the steam trap is cool and thus open to the return line. As pressure builds in the heat exchanger, the air is supposed to be pushed by the steam pressure through the air valve of the steam trap into the return line. When the steam reaches the steam trap the temperature causes the thermostatic air valve to close and the float operated valve in the steam trap then takes over, opening and closing as needed to drain away the condensate resulting from the condensing steam in the heat exchanger. If all of the air is not driven out of all the branches of the heat exchanger during this purging period it remains trapped. Trapped air in a branch prevents steam from effectively filling that branch and renders the branch incapable of heat transfer.
Accordingly, there has been a long and unfilled need in the prior art for a steam-type humidifier of the type which uses a heat exchanger to transfer thermal energy from a first steam source to heat water into steam which requires less frequent maintenance, and which is constructed to circulate the motive steam through the heat exchangers as evenly and as efficiently as possible.